1. Field of the Invention
This invention relates to a communication system and, more particularly, to a scrambler circuit in a transmitter that can randomize the logic values of a sequence of bits within a data stream sent across a transmission path. The scrambler circuit selectively scrambles only a payload section of a frame of bits to minimize low frequency jitter within the data stream and, following scrambling of only the payload, an encoder can be used to encode all sections of the frame to minimize other causes of jitter, such as DC accumulation (or baseline wander) in the data stream.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
A communication system generally contains at least two nodes interconnected by a transmission path. Each node may include both a transmitter and a receiver, generally referred to as a transceiver. The transceiver provides an interface between signals sent over the transmission path and an electronic subsystem that operates upon that signal in the digital domain. The interconnected nodes can be organized according to various topologies, such as bus, ring, star, or tree topology. The transmission path between nodes can be either wired or wireless, and it is preferred that the transmission path accommodate different types of data. For example, the path may be called upon to transfer packetized data or streaming data.
While streaming data has a temporal relationship between samples produced from a source onto the network, packetized data need not be time-related since the packets are typically stored and used later by the destination device. The streaming data can be sent either as isochronous streaming data or synchronous streaming data depending on the sample rate (fs) local to a node and the frame synchronization rate (FSY) of the transmission path. When sending a bitstream of packetized or streaming data across the transmission path, it is beneficial to know the location of such data within a payload relative to the preamble and parity sections of that frame. Thus, a frame of data sent as a data stream includes a payload section, a preamble section and, in some instances, a parity section. The preamble section can be used to generate FSY, while the parity section can be used to detect any errors in transmission and, possibly, correct those errors.
Regardless of what type of data is being sent across the transmission path or the arrangement of the payload relative to the parity and preamble sections, the transmission path and the receiver are time dispersive. In other words, the transmission path and receiver will more readily transfer certain frequency components of a data stream than other frequencies and thus their frequency response is not flat. For example, the transmission path has certain low-pass characteristics. When receiving a data stream that consists of symbols, the transmission path has the effect of widening each symbol in time, thus causing what is known as inter-symbol interference, or ISI. ISI can result in the loss or incorrect communication of certain bits within the data stream. As another example, the receiver itself might also exhibit low-pass and/or high-pass characteristics. When coupled with a phase-locked loop (PLL) at the receiver for recovering a clock signal, the additional low-pass filter within the PLL will further attenuate the higher frequency components of the data stream.
The frequency response of the transmission path, receiver, and PLL, attribute to errors in the recovery of transmitted data streams that have variations in pattern density. For example, sufficient variations in density might cause a skewing in the direct current (DC) baseline seen by a decision circuit located in the receiver. If the DC value changes over time, the decision circuit might temporarily displace the moment in which it samples a transition of the incoming data stream. Thus, the variations in pattern density which create DC accumulation, oftentimes referred to as baseline wander, can cause jitter in the output of the receiver. Moreover, if the output is used to recover a clocking signal, the jitter will be induced into the sample transitions of the clocking signal to produce bit errors in the recovered data stream.
Even in instances in which there is little if any baseline wander, jitter may still be present in the output data stream. For example, if the data stream sent across the transmission path has relatively even high density patterns relative to low density patterns, the decision circuit, oftentimes referred to as a “slicer,” operates as a comparator connected to slice the transferred data at different amplitude points along the waveform edges, and the finite rise and fall times of such edges are dependent on the sparse and dense incoming data stream transitions. If the period between sparse and dense transitions occurs at a frequency within the low-pass response of the PLL low pass filter (below the PLL low pass filter cutoff corner), then additional jitter will be imparted to the recovered output.
It would be desirable to introduce a communication system having a transmitting circuit and receiving circuit that will minimize baseline wander and modify any data dependent jitter frequency to be above the PLL low pass cutoff. The desired communication system can, therefore, minimize any low frequency jitter accumulation around a ring-based network. It would be further desirable to selectively remove such jitter from only the payload section since it is beneficial that the preamble and parity sections are to be coded. The desired preamble and parity sections are coded to synchronize the frame (and sections within the frame) and to eliminate all effects of baseline wander below the frame rate and to ensure the coded frame is DC free (i.e., has no DC accumulation as read by the receiver). Moreover, the desired communication system should be one that can accommodate all types of packetized and streaming data within a frame and, specifically, the payload section of a frame.